1. Field of the Invention
The invention relates to the securing of voice transmissions, and in particular, to an apparatus and method to improve the security of analog voice communications.
2. Problems in the Art
The present proliferation of portable communications devices has resulted in a corresponding need for security relative to those communications. Portability many times means reliance on wireless transmission, at least for a portion of the communications path. Examples are two-way radios and cellular phones. Wireless transmission, usually in free space, carries with it the problem that the communication is subject to interception. If an eavesdropper has the appropriate equipment, location and/or knowledge, the eavesdropper could relatively easily intercept the communication in as intelligible form as can the intended recipient. For example, if the interloper knew the frequency, was within transmission range, and had receiving equipment compatible with the transmitter and transmission method, the communication could be intercepted. Presently, most analog voice transmission systems are standardized and scanners allow relatively easy searching for frequencies of operating channels.
One attempted solution to this problem is to scramble the audio portion of the transmission. Manipulation of the analog representation of the voice can create an analog signal that is unintelligible to a casual eavesdropper. The transmitter and receiver, however, must both know and be synchronized to the method of manipulating the signal so that the receiver can unscramble the scrambled audio from the transmitter. Therefore, synchronization information or data must be transmitted to a receiver along with the scrambled audio (voice) that contains the voice communication intended for authorized recipients.
Examples of this type of analog scrambling are many and well-known in the art. Rolling code inversion scrambling and spectral rotation are two such examples. Examples of spectral rotation can be found in co-pending, co-owned U.S. Ser. No. 08/673,348, to inventors Burdge and Poulsen, filed Jun. 28, 1996, and co-pending, co-owned U.S. Ser. No. 08/691,600, to inventor Heermann, filed Aug. 2, 1996, respectively, both of which are incorporated by reference herein.
While scrambling of audio prevents the eavesdropper from immediately understanding the content of a voice communication, it does not necessarily mask the interception of the signal bearing the communication nor the recognition that the signal is a voice communication. If, by design or by chance, someone locked on to the frequency of transmission and listened to the signal, even scrambled certain information reveals that the signal is communicating voice or speech. For example, though unintelligible with respect to content, the received signal would reveal syllabic information. In other words, the listener would hear essentially noise, but the noise would have cadence and duration the same as speech, and would be separated by spaces (e.g. between syllables) or periods of silence, just as in speech. Therefore, at least to those of some experience in the art, most analog scrambling techniques would reveal to an interloper enough information that they could with a certain level of assurance accurately predict the probability that the signal was carrying a voice communication.
Those with enough sophistication, knowing this, could try different techniques to break the scrambling method. Thus, if trying to intercept a certain voice communication, the mere fact that an intercepted signal is probably voice is a head start. Such a priori knowledge can be important to finding and breaking the scrambled audio.
An analogy can be made with facsimile transmissions. Even though listening to the audio transmission of fax reveals no information about the content of the fax, the fax tone that is sent with each fax transmission reveals it to be a fax transmission. The interloper would have this headstart on recovering the content of the fax transmission.
There have been attempts at what will be called "masking" analog audio transmissions. An example can be seen at U.S. Pat. No. 5,101,432, incorporated by reference herein, which teaches a method for securing communications by using a Finite Impulse Response (FIR) filter with random taps. This technique tends to reduce the processed signal to a white-noise-like signal, thus masking the intended content. The intended signal is recovered at the end of the communication channel by using the inverse of the FIR filter. Thus, one trying to eavesdrop on communications would hear white noise, which would not have any information to suggest it is a voice communication. However, because this filter is by definition linear, no significant security is achieved because there is substantial prior art for recovering signals in noise. Therefore, while the masking technique, at least superficially, seems to hold promise for voice security, it in fact does not provide a substantial level of security.
A further example of masking is disclosed in U.S. Pat. No. 5,008,937 to Yamamura et al. Like U.S. Pat. No. 5,101,432 discussed above, it discloses a type of masking. It uses a generated pseudo random number (PN) sequence to create the appearance of white noise. However, if the PN sequence used to generate the "scrambling noise" becomes known, it is straightforward to break the code and recover the voice content. Thus, there is a need in the art for an improvement to analog scrambling where a higher level of security can be achieved.
It is therefore a primary object of the present invention to provide an apparatus and method for secured analog wireless voice transmission which improves over and solves the problems and deficiencies in the art.
Further objects, features, and advantages of the present invention include an apparatus and method as above-described which:
1) provide substantial, non-linear security for wireless analog communications. PA1 2) provide enhanced security over current scrambling and masking techniques. PA1 3) are non-complex. PA1 4) are economical. PA1 5) are durable. PA1 6) take away certain a priori information that can provide an eavesdropper a head start towards identifying and then descrambling or decrypting a voice communication.
These and other objects, features and advantages of the present invention will become more apparent with reference to the accompanying specification and claims.